1. Field of the Invention
This invention generally relates to gate-controlled switches, and more particularly to a gate turn-off thyristor which achieves enhanced performance through an improvement in turn-off current commutation.
2. Description of the Related Art
Gate turn-off (GTO) thyrisitor switches are the foundation of the high-power electronics industry. Structurally, GTO thyristors are four-layered, three-terminal silicon semiconductor devices which resemble a traditional thyristor in terms of performance but has the additional attribute that anode current can be turned off by control of its gate. Traditionally, a GTO""s turn-off performance is limited by the slow gate current rising rate, and the turn-off is conducted with a turn-off gain (defined as Ia/Ig at turn-off point) in the range of 3 to 5. Major drawbacks of the GTO include: 1) large gate control power due to the fact that the gate current has to be supplied for a long time; 2) slow switching speed due to long storage time; and 3) poor turn-off Safe Operation Area (SOA) due to non-uniform current distribution during turn-off. Because of the poor SOA problem, a GTO turn-off requires a dV/dt snubber, typically in the form of an R-C-D network. The RCD snubber is not only bulky, but also very lossy because each time the GTO turns on it will have to dissipate the energy stored in the snubber capacitor. The slow switching speed and high control power limits GTO""s application in high frequency Pulse Width Modulation (PWM) converters.
In recent years, the turn-off performance of GTO thyristors has been significantly improved by turning it off at the unity-gain condition (Ig=Ia at turn-off). By taking this approach, also known as hard-driven approach, the turn-off storage time has been shortened from typically several tens of microseconds to about one microsecond. The maximum turn-off current hence the SOA of these GTO thyristors has been significantly increased and the bulky and costly dv/dt snubber can be removed. High-power, hard-driven GTO thyristors can operate at frequencies of one kilohertz or above in a practical application system. See Motto et al. xe2x80x9cThe Emitter Turn-Off Thyristor (ETO) Based High Voltage, High Frequency Converter System,xe2x80x9d 1999 Center for Power Electronics Systems Annual Power Electronics Seminar, pages 340-345.
The condition known as unity gain turn-off corresponds to a situation where the cathode current is completely commutated to its gate before the end of the storage stage, or before the anode voltage increases. The essence of this is to break the PNPN latch-up mechanism, changing the GTO into an open-base PNP transistor mode during the turn-off transient. Typically, the GTO has a storage time of 1 microsecond under the unity gain turn-off condition. Therefore, the current commutating rate has to be very high (xcx9canode current/1 xcexcsec) for high-current GTOs.
Three major methods (devices) have been reported for operating GTOs in the unity turn-off gain condition. The first method was applied to an emitter turn-off (ETO) thyristor which made use of two switches to realize the high turn-off current commutation. During turn-off, its emitter switch QE is turned off while the gate switch QG is turned on. A voltage as high as the breakdown voltage of QE can be applied on the gate loop stray inductor LG, realizing fast current commutation. See Li et al., xe2x80x9cIntroducing the Emitter Turn-Off Thyristor (ETO),xe2x80x9d IEEE Industry Applications Society 33rd Annual Meeting, Oct. 12-15, 1998, pages 860-864.
The second method was applied to a MOS turn-off (MTO) thyristor. This device was implemented using only a gate switch QG During turn-off, the gate switch is turned on, bypassing the current through the GTO gate-cathode diode and realizing unity gain turn-off. See Piccone et al., xe2x80x9cThe MTO Thyristorxe2x80x94A new High Power Bipolar MOS Thyristor,xe2x80x9d IEEE Industry Applications Society 31st Annual Meeting, Oct. 6-10, 1996, pages 1472-1473.
The third method was applied to an integrated gate commutated thyristor (IGCT) . This IGCT uses one voltage source and one gate switch. By dramatically decreasing the gate loop stray inductance, a high current commutation rate is achieved in the IGCT.
Each of the aforementioned devices has inherent limitations which degrade performance. The emitter switch of the ETO thyristor needs to conduct the main current, so it is implemented by paralleling many MOSFETs. The turn-off voltage for the current commutation in the MTO thyristor is less than the forward voltage drop of the GTO gate-cathode diode, so the current commutation rate is low.
Insofar as the IGCT is concerned, it is indisputable that the turn-off current commutation diG/dt is crucial to the performance of this type of device. As has been reported in the previously referenced Yamamoto article, even under the unity gain turn-off condition, higher commutation di/dt ensures a more uniform turn-off transient process over the entire GTO thyristor wafer. This, in turn, increases the turn-off capability of the device. Nevertheless, conventional IGCT devices remain limited by the turn-off loop stray inductance and the maximum turn-off voltage, the latter of which is typically about 20 V.
More specifically, in conventional IGCT devices a further increase of the turn-off current commutation diG/dt is difficult because of its gate loop stray inductance LG and the maximum available voltage source VOFF that can be used. For example, the gate loop stray inductance of a 4-inch IGCT has been reduced to 3 nH, which is considered to be very low and is even harder to decrease. The maximum gate turn-off voltage that can be applied to the IGCT has to be less than the breakdown voltage of the GTO""s gate-cathode diode. Because this breakdown voltage is typically about 20 V, a maximum of about 6 kA/xcexcsec commutating di/dt is achievable. See Yamamoto et al., xe2x80x9cGCT (Gate Commutated Turn-Off) Thyristor and Gate Drive Circuit,xe2x80x9d PESC 1998, pages 1711-1715. The use of such a low maximum turn-off voltage has adversely affected the performance of the IGCT.
In view of the foregoing discussion, it is clear that a need exists for an improved gate turn-off thyristor which has fewer limitations than its conventional counterparts, and more specifically one in the form of an integrated gate commutated thyristor which operates based on a higher maximum turn-off voltage and thus with enhanced performance than conventionally possible in IGCT devices.
It is a first object of the present invention to provide a gate turn-off thyristor switch which has improved performance compared with conventional gate turn-off thyristor switches.
It is a second object of the present invention to achieve the first object by increasing the turn-off current commutation diG/dt to values above that achievable by conventional IGCT, ETO, and MTO devices.
The foregoing and other objects of the invention are achieved by providing a gate turn-off thyristor circuit which, in accordance with one embodiment, includes a gate turn-off thyristor connected to an anode, a diode connected in series with the gate turn-off thyristor and a cathode, a turn-off voltage source connected to the diode, and a gate loop stray inductance element connected between the turn-off voltage source and the gate turn-off thyristor. In this arrangement, the diode is a discrete diode. Its breakdown voltage may therefore be selected to be a very high value. This, in turn, allows for use of a high turn-off voltage Voff to achieve a very high turn-off current commutation diG/dt which enables the GTO thyristor circuit of the present invention to achieve enhanced performance while maintaining a relatively simple circuit design. Also, the thyristor circuit of the present invention operates with equivalent performance as an integrated gate commutated thyristor in the aspect of unity turn-off gain, fast turn-off transient, and snubberless turn-off capability.
A second embodiment of the thyristor circuit of the present invention resembles the first embodiment except that a plurality of diodes are connected in parallel between the gate turn-off thyristor and a cathode terminal.
A third embodiment of the thyristor circuit of the present invention resembles the first embodiment except that a single diode wafer is packaged with the GTO inside the GTO""s press-pak.
High current/power semiconductor devices are crucial to modern industry, and gate turn-off thyristors are the only practical candidate. The thyristor of the present invention significantly improves the performance of high-current GTOs, leading to improved power electronics and industry application. Furthermore, the invention is economically feasible to implement in terms of cost and ease of design.